The system and method disclosed herein relates to printing systems that generate images onto continuous web substrates. In particular, the disclosed embodiments relate to control of the cross-process control of printheads in such systems.
Printers provide fast, reliable, and automatic reproduction of images. The word “printer” as used herein encompasses any apparatus, such as a digital copier, book marking machine, facsimile machine, multi-function machine, etc., which performs a print outputting function for any purpose. Printing features that may be implemented in printers include the ability to do either full color or black and white printing, and printing onto one (simplex) or both sides of the image substrate (duplex).
Some printers, especially those designed for very high speed or high volume printing, produce images on a continuous web print substrate. In these printers, the image substrate material is typically supplied from large, heavy rolls of paper upon which an image is printed instead of feeding pre-cut sheets from a bin. The paper mill rolls can typically be provided at a lower cost per printed page than pre-cut sheets. Each such roll provides a very large (very long) supply of paper printing substrate in a defined width. Fan-fold or computer form web substrates may be used in some printers having feeders that engage sprocket holes in the edges of the substrate.
Typically, with web roll feeding, the web is fed off the roll past one or more printhead assemblies that eject ink onto the web, and then through one or more stations that fix the image to the web. A printhead is a structure including a set of ejectors arranged in at least one linear array of ejectors, for placing marks on media according to digital data applied thereto. Printheads may be used with different kinds of ink jet technologies, such as liquid ink jet, phase-change ink, systems that eject solid particles onto the media, etc.
Thereafter, the web may be cut in a chopper and/or slitter to form copy sheets. Alternatively, the printed web output can be rewound onto an output roll (uncut) for further processing offline. In addition to cost advantages, web printers can also have advantages in feeding reliability, i.e., lower misfeed and jam rates within the printer as compared to high speed feeding of precut sheets through a printing apparatus.
A further advantage is that web feeding from large rolls requires less downtime for paper loading. For example, a system printing onto web paper supplied from a 5 foot diameter supply roll is typically able to print continuously for more than an hour at speeds of about 500 feet per minute (fpm) without requiring any operator action. Printers using sheets, which usually print at speeds of about 100 fpm, may require an operator to re-load cut sheet feeders 2 to 3 times per hour. Continuous web printing also provides greater productivity for the same printer processing speed and corresponding paper or process path velocity through the printer, since web printing does not require pitch space skips between images as is required between each sheet for cut sheet printing.
To achieve the high speeds desired in continuous web printing and to cover the width of the web as required in production printing, multiple printheads are used. As the printer operates, the printheads expand and contract in response to changing thermal conditions. Thus, the width covered by a particular printhead (the “extent” of the printhead) varies depending on the operating temperature. Likewise, the rolls used to define the process path expand and contract in response to temperature changes. The expansion and contraction of the rolls affects the alignment of the process path. Likewise, the paper media expands and contracts as moisture leaves the paper at varying rates as the local temperature changes throughout the process. “Alignment” as used herein, unless otherwise expressly qualified, is defined as the location of the printhead along the width of the process path immediately adjacent to the printhead (cross-process location), and the orientation of the cross-process axis of the printhead with respect to an axis perpendicular to the edge of the process path. Thus, the web, which is designed to move perpendicularly past each of the printheads along the in-track axis of the process path, may move past a printhead at a skewed angle or may be displaced in the cross process direction when the printhead is misaligned with respect to the web. Additionally, the cross-process extent of the printhead may not be positioned properly with respect to the other printheads.
Misalignment resulting from movement of the printheads and the rolls is exacerbated by the positioning of printheads for different colors at different locations along the in-track axis of the process path. Specifically, printers that generate color copies may include one or more printheads for each color of ink used in the printer. Each of the printheads associated with the different colors is positioned at a location along the in-track axis of the process path that may be separated from other printheads by one or more roll pairs. Each roll pair produces a unique alignment of the media with respect to the process path. Accordingly, changes in the printheads and rolls may cause the printheads to be misaligned with the web as it moves along the process path.
Alignment of printheads to account for the changes caused by thermal expansion and contraction of the printheads (static alignment errors) is known. The correction of static alignment errors increases the clarity of images produced on the web. The clarity that can be obtained, however, is limited by the introduction of dynamic alignment errors, which are manifested during operation of the printing system. These dynamic errors are not corrected by the alignment of the printheads to account for thermal expansion and contraction of the printheads. Consequently, alignment procedures for printing systems, which reduce dynamic errors, would be beneficial.